Peridots are very affordable gems. The price depends on their quality (color/cut/clarity) as well as their size. If you are looking for a ring size Peridot, you're probably looking at stones of about 1ct to 10cts. Here is the price range (USD/ct) for Peridot stones of this size as listed in the Spring/Summer 2006 Guide *:-
Extra Fine 60-85
Extra Fine 100-150
Unless you have been trained to grade stones, it will be difficult for you to be able to tell apart a good stone from an extra fine stone. My advice would be to see a trusted gem dealer or jeweler. In general, look for clean stones that are well proportioned with a good finish. Pure green is ideal ("summer grass green"), followed by Yellowish green and then last on the scale, Greenish yellow.
Do be careful with a Peridot ring. Peridots are not very hard (6.5 on the MOH's hardness scale vs 10 for Diamond and 9 for Ruby & Sapphire), can be a tad brittle and susceptible to acids (creams, hair products).
* The Guide publishes wholesale gem prices in the US. Check www.gemguide.com for subscripions and more information.
I lOVE gemstones!